The effects of high concentration of ammonia nitrogen on the gill tissue structure of yellow catfish ( <i>Tachysurus fulvidraco</i> )

Ammonia nitrogen is a ubiquitous environmental stressor in aquaculture systems, posing severe toxicity to aquatic organisms. This study investigated the histopathological and physiological responses of Tachysurus fulvidraco gills under acute ammonia stress. We exposed fish to gradient concentrations of ammonia nitrogen and analyzed gill tissue histology, serum ion concentrations, blood ammonia levels, and inflammatory markers. Results showed a 96h LC50 of 28.12 mg/L. Histopathological analysis revealed severe gill damage, including lamellar fusion, epithelial necrosis, and inflammatory cell infiltration, linked to NF-κB-mediated upregulation of inflammatory factors and altered expression of tight junction proteins (occludin, ZO-2). Concurrently, serum Na⁺ and K⁺ levels significantly decreased, while blood ammonia rose dose-dependently. These findings demonstrate that acute ammonia stress disrupts gill structural integrity and osmoregulatory function through inflammation and barrier dysfunction, highlighting its role in ammonia toxicity mechanisms and providing insights for aquaculture health management.